There has been practically used a load control system for bulldozers that performs load control to make the actual tractive force of the vehicle body coincide with a preset target tractive force, in other words, to make the load on the blade constant in order to reduce the burden imposed on the operator who manipulates the blade of the bulldozer during dozing operation.
The above load control system, however, reveals the inability to stably control the load because the blade often takes an abrupt action for example when excavation starts or when one drive mode is switched to another. The inventors have made an attempt to solve this problem and proposed a load control system for bulldozers in Japanese Patent Publication No. 7-54374(1995) according to which if there is a difference between the actual tractive force and a target tractive force at a start of excavation in an automatic drive mode, the target tractive force is gradually increased or decreased thereby to eliminate the abrupt action of the blade.
The load control system disclosed in the above publication has the problem that when a blade control is carried out during excavation of soft soil, the blade suddenly cuts into the ground so that the load control and therefore the smooth excavation are hampered. Also, the system cannot control the blade to properly cut into the ground at a start of excavation of hard soil. In such situations, there arises a need for the intervention of manual control by the operator (manual control intervention), which has been an obstacle to the introduction of automatic control.
The invention is directed to overcoming the above problems and one of the objects of the invention is therefore to provide a dozing system for use in a bulldozer, which enables the application of automatic control to excavation of all types of soil properties and reduces the frequency of manual control intervention.